The Norwalk virus and related 27nm caliciviruses are an important cause of epidemic viral gastroenteritis that occurs in family, school, group, institutional, or community-wide outbreaks affecting adults, school-aged children, family contacts, and some young children as well. Most information concerning the epidemiology of this group of viruses has been generated using diagnostic assays that employed either native Norwalk virus antigen present in or purified from human stools or, more recently, recombinant Norwalk VLPs to detect serologic responses. Epidemiologic studies were conducted this year using newly developed diagnostic assays that employed recombinant virus-like particles (VLPs) from the Hawaii, Toronto, or Desert Shield viruses that are serotypically distinct from Norwalk virus. The association of several gastroenteritis outbreaks, including those in the elderly in a Maryland nursing home study, with Hawaii virus or Toronto virus indicates that a better understanding of the epidemiology of caliciviruses will emerge from this approach. In addition, the VLPs and specific hyperimmune sera raised against the VLPs were used in immune electron microscopy (IEM) studies to examine the antigenic relationships among human caliciviruse.